Currently, photocopiers have been required to be able to copy an image on a large number of sheets at high speed with small size thereof, while maintaining high image quality. Conventional high-speed photocopiers however have not necessarily achieved downsizing. One of the reasons for this is a space required for residual toner collected after transferring. Meanwhile, a treatment of the residual toner collected after transferring is very serious issue in view of current environmental problems. Namely, the aforementioned problems can be solved, and a small and high speed photocopier without causing environmental problems can be achieved by supplying the residual toner collected after transferring to a developing unit. As a result of the supplement of the toner, a number of sheets on which copying can be performed increases, cost per one operation of copying reduces, and therefore the entire process becomes economical.
In the past, there has been an attempt to use the residual toner collected after transferring in a developing step by supplying the residual toner to a developing unit. However, various problems occur when this step is introduced, such as deterioration of images, and reduction in image density, as copying on a large number of sheets is repeatedly performed, and therefore there is a problem that it is difficult to stably provide images over a long period.
PTL 1 discloses a developer that intends to solve the aforementioned problems by regulating a particle size distribution of a toner.
Specifically, the developer uses toner particles where 90% by mass or greater of the whole toner particles is in the range of D(3√2)−1 to 3√2D, and a proportion of the toner particles smaller than D(3√2)−1 is 5% by mass or smaller, when the volume average particle diameter of the toner is D (μm).
However, application of this technique is limited to a two-component developing method. Since a proportion of extremely small particles is kept low, there are advantages that defects such as fogging and toner scattering are prevented at the time of recycling, but on the other hand, accurate copy of a precise latent image cannot be produced as the proportion of the small particles is extremely low.
Moreover, PTL 2 discloses a toner having a certain particle size distribution, but which has not yet achieved prevention of carrier spent, or fogging caused by the recycled toner.
Meanwhile, in the course of image formation by electrophotography, a toner is fixed onto an image support by a thermal fixing method to obtain a permanent visible copy image.
In the case where a copy image is formed on a transparent sheet for an overhead projector (OHP), such sheet may be referred to as an “OHP sheet” hereinafter, with a toner, especially a color toner, it is required to fix an image to give a smooth image surface to thereby prevent scattering of transmitting light, or diffused reflection on the image surface at the time of projecting, for the purpose of attaining excellent light transmittance of a projected image by the OHP.
To this end, as a common conventional method, used is a color toner, which has low viscoelasticity at a melting point thereof compared to a conventional black toner, and can be rapidly transferred into a melted state, so that an image surface is easily smoothed by heating and pressurizing.
However, making viscoelastic properties of the toner low in the aforementioned manner also lowers the glass transition temperature of the toner at the same time, and therefore, the dynamic strength of the toner lowers at ambient temperature, specifically when the toner is used in a device. Accordingly, a problem that an external additive on a surface of a toner particle is embedded to thereby deteriorate developing properties and transferring properties occurs, for example, by application of physical stress, such as stirring, within a developing unit.
Moreover, toner spent, which is deposition of toner particles on carrier particles, occurs.
These problems are significantly caused when toner particles are made small for achieving high quality images to meet current needs. This is because the toner tends to receive physical stress more easily as particle diameters of the toner particles become smaller.
To solve the aforementioned problem, in PTL 3, a toner, which has a certain relationship between the volume average particle diameter Dv (μm) and storage elastic modulus G′170 (dyne/cm2) at 170° C., is used to improve transparency with an OHP. On theory, such toner increases its storage elastic modulus as the average particle diameter of the toner decreases. When the average particle diameter of the toner is small, it is disadvantageous in achieving low temperature fixing ability and desirable glossiness of the toner. As a result, color reproducibility may be lowered. Namely, it is not easy to achieve all of the current needs, which are a toner of small particle diameters, high image quality, low temperature fixing ability, and color reproducibility.
As measure for improving low temperature fixing ability of a toner, PTL 4 discloses a binder resin for a toner, which contains non-crystalline polyester, and crystalline polyester that has a significant effect for improving low temperature fixing ability, compared to conventional non-crystalline polyester. In the case where the crystalline polyester and the non-crystalline polyester are used in combination, however, transesterification occurs during a melt-kneading process because compositions of the both resins are similar. Therefore, high crystallinity of the ctystalline polyester cannot be maintained, leading to low shelf stability of a resulting toner.
PTL 5 and PTL 6 each disclose a binder resin for a toner, which contains crystalline polyester using sebasic acid or adipic acid as a carboxylic acid component, and a styrene-acryl resin, and evaluate shelf stability under low temperature, and fixing ability at low printing speed. However, further improvements of their performances are desired.
PTL 7 discloses a binder resin for a toner, which contains certain crystalline polyester, and a non-crystalline hybrid resin, and discloses that a toner having a wide fixable temperature range and durability can be provided. However, it has not solved fogging and uneven image density caused by poor compatibility between the crystalline polyester and a pigment.
PTL 8 discloses use of crystalline polyester, as a binder resin, to achieve low temperature fixing ability, where the crystalline polyester resin contains the structure represented by —OCOC—R—COO—(CH2)n— (with proviso that R is a C2-C20 linear-chain unsaturated aliphatic group, and n is an integer of 2 to 20) in an amount of 60 mol % relative to the total ester bonds in the entire resin, but it has not discussed about improvement of shelf stability.
Meanwhile, it is proposed that a toner container is composed of a flexible material and as well as the toner, the toner container is also made compact, such as by folding, after use for collection, in view of environmental concern. Such container has a large rate of volume reduction, and can be achieved at low cost. Moreover, such container can automatically supply a toner by air after loaded in a main body of a device so that a toner is not flown in the air.
Such container, however, has problems including (1) toner supply is not stable; (2) there are cases where a toner cannot be supplied because the toner is packed in the container after a period of storage; and (3) a residual amount of the toner is large. These are caused because a spiral groove for supplying a toner cannot be formed in the container, a member for supplying a toner, such as an agitator, cannot be incorporated into the container, etc.